Touch sensing systems (“touch systems”) are in widespread use in a variety of applications. Typically, the touch systems are actuated by a touch object such as a finger or stylus, either in direct contact, or through proximity (i.e. without contact), with a touch surface. Touch systems are for example used as touch pads of laptop computers, in control panels, and as overlays to displays on e.g. hand held devices, such as mobile telephones. A touch panel that is overlaid on or integrated in a display is also denoted a “touch screen”. Many other applications are known in the art.
WO2011/028169 and WO2011/049512 disclose touch systems that are based on frustrated total internal reflection (FTIR). Light sheets are coupled into a panel to propagate inside the panel by total internal reflection (TR). When an object comes into contact with a touch surface of the panel, the propagating light will be attenuated at the point of touch. Arrays of light sensors are located around the perimeter of the touch surface to detect the received light for each light sheet. Measurement signals from the light sensors may be repeatedly processed for input into an image reconstruction algorithm that generates a two-dimensional distribution of attenuation values across the touch surface. This enables repeated determination of current position/size/shape of touches while one or more users interact with the touch surface.
In these types of FTIR-based touch systems, the touches need to be detected against a background of interferences, e.g. originating from fingerprints and other types of smear on the touch surface. The influence of interferences may vary not only over time but also across the touch surface, making it difficult to properly detect the touches on the touch surface at all times. Furthermore, the degree of interaction between a touching object and the touch surface may vary both over time and between different objects. For example, the interaction may depend on if an object is tapped, dragged or held in a fixed position onto the touch surface. Different objects may yield different degree of interaction, e.g. the degree of interaction may vary between fingers of a user, and even more so between the fingers of different users. It is also to be understood that a touching object may result in only a small attenuation of the propagating light, e.g. less than 1%. Certain systems may need to be designed to detect attenuations on the order of 0.1%-0.01%.
The above-mentioned WO2011/049512 proposes a technique of compensating for contaminations on the touch surface. In one embodiment, the received light energy at the light sensors is converted into attenuation values, e.g. by normalization with a reference value for each light sensor, whereupon the attenuation values are input into an image reconstruction algorithm that generates a current light status, which is a two-dimensional distribution of the attenuation values across the touch surface. The touch system also keeps track of a background status, which is a two-dimensional distribution of attenuation values caused by contaminations on the touch surface. Touches are then detected in a compensated light status which is generated by subtracting the background status from the current light status. To be able to detect a touch, represented as a small attenuation in the current light status, it may be necessary to implement the reconstruction algorithm so as to generate the current light status with a high bit resolution in attenuation values. However, this is demanding in terms of processing, and may lead to significant processing times. It is also conceivable that the available bit resolution for the reconstruction processing may be limited by hardware constraints.
The above-mentioned WO2011/028169 proposes an alternative compensation technique. The reference values for the light sensors, collectively denoted a background signal profile and used in the normalization and conversion of the measured energy values into attenuation values, are intermittently updated so as to include the influence of contaminations on the touch surface. The touch system repeatedly reads the energy values from the light sensors and uses current (updated) reference values to generate attenuation values, collectively denoted a current compensated signal profile. The current compensated signal profile may then be processed for touch determination, e.g. by means of an image reconstruction algorithm that generates a two-dimensional distribution of attenuation values across the touch surface, which may be further processed for touch determination. By tracking the influence of contaminations via the updating of the reference values, the touch system compensates for the contaminations already in the input to the reconstruction algorithm. Theoretically, by compensating on the input side, it is possible to operate the reconstruction algorithm with a reduced bit resolution while being able to detect small changes in attenuation in the reconstructed image. One challenge, however, is to achieve a sufficiently adequate compensation on the input side. If attenuation from contaminations remains in the reconstructed image, touch determination may be more or less hampered.